A technique to culture epithelial rabbit endometrial cells in chemically defined medium has been developed. Estrogens were found to promote cell division, while progesterone had an opposite effect. These cultured cells were found to be either noncycling (G0) or cycling. Both cell subpopulations were isolated and it was found that the G0 cells were the target of estrogens. Interaction between progesterone and the cycling cells was necessary to obtain an antagonistic effect on the estrogen action on the G0 cells, apparently due to the production of a diffusible factor. A similar factor, probably a protein, appears to be produced by high density cultures, which do not respond to estrogen stimulation. Estradiol and progesterone decrease the number of estrogen receptors in cultures, apparently by different mechanisms of action. We will continue to study our experimental system by analyzing the mechanism of action of ovarian sex hormones, epidermal growth factor and prostaglandin F-2alpha in regulating proliferation, cell cycle, blastokinin synthesis, and hormone receptors. The endpoint of our research is to improve our comprehension of the mechanism by which sex hormones regulate proliferation and differentiation of endometrium. It is expected that the study of our hormone-responsive system will help to advance the understanding of the etiology and pathogenesis of endometrial carcinoma in particular and hormone-related cancer in general.